1. Field of the Invention
The present invention relates to a catalyst system for the treatment of exhaust gases from diesel engines.
2. Description of the Related Art
The exhaust gas from diesel engines contains, during normal operating phases, a high proportion, about 3 to 10 vol. %, of oxygen in addition to unburnt hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x). Since the concentration of oxygen in the exhaust gas is greater than that required stoichiometrically, it is not possible to convert all three hazardous substances by the three-way process conventionally used with petrol engines. Petrol engines usually operate with normalised air/fuel ratios .lambda. close to 1, while diesel engines operate with normalised air/fuel ratios greater than 1.2. The normalised air/fuel ratio .lambda. is the air:fuel ratio (kilograms of air to kilograms of fuel) standardized to stoichiometric operation.
The composition of the exhaust gas from diesel engines is very dependent on the particular operating phase of the engine. In the cold-start phase, that is the first 60 to 120 seconds after starting the engine, the exhaust gas has a high concentration of hydrocarbons, but the concentration of nitrogen oxides is still low. With longer operating times and under higher engine loads, the emissions of unburnt hydrocarbons decrease and emissions of nitrogen oxides increase.
The unburnt hydrocarbons and carbon monoxide in diesel gas exhausts can be converted relatively easily by oxidizing catalysts.
This type of catalyst is described, for example, in DE 39 40 758 C2. This comprises an oxidizing catalyst with high rates of conversion for hydrocarbons and carbon monoxide and an inhibited oxidizing effect towards nitrogen oxides and sulfur dioxide. Nitrogen oxides can pass over the catalyst virtually unchanged.
Nitrogen oxides can be converted only by using special reducing catalysts, due to the high oxygen content of diesel exhaust gas. Basically this means that these reducing catalysts also exhibit a high oxidizing effect towards carbon monoxide and hydrocarbons.
The rates of conversion of a reducing catalyst for the individual hazardous components depend strongly on the exhaust gas temperature. With increasing exhaust gas temperature, the oxidation of hydrocarbons and carbon monoxide is initiated first and oxidizing rates of more than 70% are achieved within a temperature interval of about 150 to 175.degree. C. As the temperature increases further the conversion of hydrocarbons remains approximately constant. The exhaust gas temperature at which a rate of conversion of 50% for the particular hazardous substance is achieved is called the light-off temperature for this hazardous substance.
The rate of conversion for nitrogen oxides varies in the same way as the rate of conversion for hydrocarbons. However, it does not increase regularly, but passes through a maximum at temperatures at which the oxidation of the hydrocarbons has virtually reached its maximum value and then decreases with increasing temperatures to almost zero. Optimum rates of conversion for nitrogen oxides are therefore achieved only in a narrow temperature window. The hydrocarbons and carbon monoxide contained in the exhaust gas are required as reducing agents.
The conversion curves for the individual hazardous substance depend strongly on the formulation of the particular catalyst. This also applies to nitrogen oxides. The position and width of the temperature window and the maximum degree of conversion which can be achieved in this window depend on the catalyst formulation. So-called low temperature catalysts have been disclosed and reach their maximum nitrogen oxide conversion at temperatures between 200 and 250.degree. C. In the case of high temperature catalysts, the maximum for nitrogen oxide conversion is situated above 300.degree.C.
Reducing catalysts in the prior art have a maximum conversion for nitrogen oxides in oxygen-containing diesel exhaust gas of more than 55% at a temperature of about 200.degree. C. The full width at half-maximum of the reaction curve for nitrogen oxides is about 100.degree. C.
These types of catalysts are described, for example, in "Design Aspects of Lean NO.sub.x Catalysts for Gasoline and Diesel Engine Applications" by Leyrer et. al. in SAE No. 952495, 1995, and in "Catalytic reduction of NO.sub.x with hydrocarbons under lean diesel exhaust gas conditions" by Engler et. al. in SAE No. 930735, 1993. Catalysts based on zeolites which may be exchanged with a variety of catalytically active metals (for example copper or platinum) are used. Further reducing catalysts are described in patent application DE 196 14 540.6 which is not a prior publication.
The strong temperature dependence of the rates of conversion of nitrogen oxides represents a major problem during the purification of diesel exhaust gases because the engine outlet temperature of the exhaust gases from diesel vehicles during operation can vary between about 100 and 600.degree. C. depending on the actual driving conditions. High rates of conversion are therefore only achieved during brief phases of operation during which the exhaust gas temperature is within the optimum range for the catalyst used.
DE 40 32 085 A1 discloses a catalyst arrangement for reducing nitrogen oxides in an exhaust gas which is produced within a wide range of exhaust gas temperatures. The arrangement consists of at least two catalyst beds which are arranged directly one after the other and consist of different catalyst materials which have their greatest catalytic effect in different, adjacent zones of the exhaust gas temperature range. The catalyst bed in which the optimum effect is achieved at a higher temperature is arranged upstream of the other catalyst bed.
Furthermore, DE 39 40 758 C2 discloses the treatment of exhaust gas from diesel engines by using oxidizing catalysts. For space reasons, the volume of catalyst which is required is frequently divided between one catalyst in the engine compartment and one catalyst in the under-floor region. The use of reducing catalysts has also been disclosed. To improve the nitrogen oxide conversion, additional diesel fuel, as a reducing agent, is frequently injected into the exhaust gas upstream of the catalyst.
DE 36 42 018 describes this type of arrangement. In order to remove the carbon monoxide and excess hydrocarbon which is not consumed during nitrogen oxide conversion by oxidation, the reducing catalyst may be connected to a second catalyst which may be designed as a simple oxidizing catalyst.
Hydrocarbons and carbon monoxide can be efficiently converted by known catalyst systems. The conversion of nitrogen oxides, however, is still unsatisfactory. It reaches satisfactory values only during specific operating phases of the engine.